The present invention relates to a light source apparatus for separating a white light a light having a plurality of colors and, for example, to a light source apparatus for preferably using in a projection color image display apparatus in which a matrix type passive display element is used, and more particularly, to a light source apparatus applicable to a projection color television receiver of a large screen and an apparatus for displaying information.
First, a projection color image display apparatus to which the present invention is applicable will be described. A passive type display element for use in the projection color image display apparatus to which the present invention is applicable does not itself emit a light, but the transmittance or the reflectance thereof change according to a driving signal and displays images and characters by modulating the intensity of a light emitted by a light source. The passive type display element may be constructed by a liquid crystal display element, electrochromic display, or display elements made of light-transmittable ceramic such as a PLZT (light-transmittable ceramic made of lead, lanthanum, zircon, and titanium). Above all, the liquid crystal display element is widely utilized in a pocketable TV, a word processor, and the like. When the liquid crystal display element is used, a comparatively small display element is used in a direct sight type in these apparatii in which the diagonal line of the screen is approximately 2" to 10" (inch is used as unit).
With regard to a display on a large screen more than 40", conventionally, a projection type TV for projecting an image displayed on a cathode-ray tube has been used, however, there is the disadvantage that the TV has a limit in the brightness of an image which can be displayed on the cathode-ray tube, so that there is a problem that the greater the enlargement ratio of a projected screen becomes, the darker the screen becomes. In addition, a display apparatus in which the diagonal line of the screen is approximately tens of inches is as heavy as tens of kilograms.
As a substitution thereof, a projection type TV (television receiver) using a liquid crystal element which has been developed is already commercially available. The liquid crystal element does not itself emit a light, so that it is necessary to provide a light source. However, it has a feature that in principle, the brightness of a screen can be increasingly displayed depending on the degree of the brightness of the light source. Further, compared with the projection type TV of the cathode-ray tube system having the same screen size, the projection type TV which uses the liquid crystal element has a feature that it is compact and light, so that a further development is expected.
The projection color image display system which uses such a liquid crystal display element includes the system which uses only one liquid crystal element and the system which uses three liquid crystal elements.
Similarly to the liquid crystal TV of the direct sight type, according to the former, a liquid crystal display element is provided with a mosaic-shaped color filter and is supplied with light by an optical system similar to a slide projector. This system is disclosed, for example, in Japanese Laid-Open Patent Publication No. 59-230383. This system has a simple construction of its optical system and the number of liquid crystal display elements used is only one. Consequently, it is suitable for a compact projection system. But according to this system, approximately 2/3 of a light irradiated to the liquid crystal element is absorbed by the color filter. For example, a red color filter which corresponds to a red picture element absorbs the green and blue light components. Therefore, only 1/3 of a light emitted by the light source is utilized, so that compared with a system (described below) which uses three liquid crystal display elements, the screen is darker in an apparatus which adopts one liquid crystal display element supposing that the same light source is used by both systems.
According to the system which uses three liquid crystal display elements, means for generating the light components of the three primary colors of red, green, and blue are provided separately for each display element which form an image by controlling the light components of red, green, and blue, so that a full color display is accomplished by optically superimposing the image of each color. Japanese Laid-Open Patent Publication No. 60-3291 discloses an example of such a system which uses light sources for each of the three primary colors and color filters applicable thereto. It is possible to separate a light emitted by a single white light source into light components of the three primary colors by dichroic mirrors and irradiate the light component of each color to a liquid crystal display element corresponding to each color. This system is disclosed in Japanese Laid-Open Patent Publication No. 60-179723. The dichroic mirror used in this system is formed by the combination of a multilayer film formed by a known thin film forming technique for forming a dichroic mirror on a transparent substrate such as a glass which selectively reflects or transmits a light of a certain wavelength, and a wavelength selecting multilayer film formed on the surface of a prism like a dichroic prism for use in an imaging device of a color TV. Both multilayer films are hereinafter referred to as "a dichroic mirror" in the description of the known art. Halogen lamp, xenon lamp, metal halide lamp or the like can be used as the white light source. The emission spectrum thereof may be a continuous spectrum or a bright line spectrum.
An image formed by each display element is projected on a screen through a projection lens, and there are two projection systems depending on image-viewing side with respect to the screen. One is a front projection system in which using a reflecting type screen, the image is viewed on a projection side and the other is a rear projection system in which using a semi-transparent screen, the image is viewed from the side opposite to the projection side.
In the rear projection system, since a projecting section and a screen are accommodated in one case, the distance between the projection lens and the screen is constant. The front projection system is divided into two groups. One group consists of when the system is built in, the distance between the projection lens and the screen is constant. The other group is similar to a slide projector, the distance between the projecting section and the screen is changeable. When the distance between the projection lens and the screen is constant, it is unnecessary to adjust the focus of the projection lens after by setting the focus point when it is manufactured, so that by providing each display element of the three primary colors with a projection lens, the image of each of the three primary colors can be synthesized directly on the screen through the projection lens. Japanese Laid-Open Patent Publication No. 61-167297 discloses an example of this type. On the other hand, when the distance between the projecting section and the screen is changeable, the distance therebetween is not constant. Therefore, in this system by providing each display element of the three primary colors with a projection lens as described above, the image of each of the three primary colors are synthesized on the screen through the projection lens. A user is required to adjust the focus of the projection lens and the superimposition of images for each use, thus resulting in imposing on a user this additional burden of operations. The following system is adopted to avoid this: the system for synthesizing, in the projecting section, light components which have passed through a display element corresponding to each of the three primary colors and projecting images on the screen by using a common projection lens. In this case, the user has only to adjust the focus of one projection lens. An example of this type is disclosed in Japanese Laid-Open Patent Publication No. 60-3291.
The above description is the outline of the projection color image display apparatus which uses the passive type display element.
Of the projection color image display apparatii which use the passive type display element as described above, the present invention is applicable to the projection color image display apparatus which uses three display elements. In this projection color image display apparatus, it is advantageous in view of power consumption and caloric value to separate light emitted by a single white light source into light components of the three primary colors of red, green, and blue. As known in the art, FIG. 8 illustrates a white light emitted by a light source 1 which is guided through a condenser lens 3 and plane dichroic mirrors 2b1, 2b2, 2r1, and 2r2. Additionally, reflecting mirrors 5, 6, and 7 are provided, and liquid display display elements 9r, 9g, and 9b for each color are provided. A light which has passed through the projection lens 4 is projected on a screen. The plane dichroic mirrors 2b1 and 2b2 selectively reflect the wavelength band of blue and the plane dichroic mirrors 2r1 and 2r2 selectively reflect wavelength band of red.
According to the system of FIG. 8, it is difficult to shorten the space between the light source 1 and each of the display elements 9r, 9g, and 9b because two dichroic mirrors 2b1, 2b2 and 2r1, 2r2 in approximately the same size as that of each of the display elements 9r, 9g, and 9b are inserted between the condenser lens 3 and the display elements 9r, 9g, and 9b with the two dichroic mirrors at an angle of 45.degree. to the optical axis of the condenser lens 3. Thus, the system has a problem that if the light from the light source is not collimated well, light is dispersed before it reaches the display elements, so that an amount of light to be effectively utilized is reduced.
In addition, the system of FIG. 8 has the following problem: That is, the dichroic mirrors are used in an incident angle of approximately 45.degree., so that in this case, compared with the case in which a light is incident on the dichroic mirror perpendicularly, the degree of dependency on an incident angle of the spectral characteristic is great. Therefore, if the light from the light source is not collimated well, color nonuniformity (nonuniformity of white balance) is likely to occur on the screen.
In order to overcome this problem, Japanese Laid-Open Patent Publication No. 61-99118 discloses dichroic mirrors arranged in orthogonal X-type to be used as means for separating light emitted by a single white light source into light components of the three primary colors of red, green, and blue and for synthesizing the image, of each of the three primary colors, formed by each display element. As an optical apparatus which uses dichroic mirrors arranged in the orthogonal X-type, Japanese Laid-Open Patent Publication No. 50-10019 discloses "a projection type television receiver for optically synthesizing images, formed by three cathode ray tubes which emit three primary colors, through dichroic mirrors arranged in the orthogonal X-type".
However, the dichroic mirrors arranged in the orthogonal X-type are composed of X-configured two pairs of dichroic mirrors formed by separating two dichroic mirrors into two, respectively although a dichroic mirror should essentially be jointless. Therefore, the spectral characteristic and the reflection angle thereof are likely to discontinue in jointed portions, so that seams appear on the display screen and hence the display quality degrades.